Posted by Steve in VA on January 29, 2016 at 01:13:57 from (72.73.44.249):
In Reply to: HEAT OF VAPORIZATION posted by Geo-TH,In on January 28, 2016 at 14:33:39:
Yes, it is 3:45 in the morning but I've been pondering this issue and I believe that attributing this frosting to LHoV is incorrect. Please forgive me if I drone on. LHoV is the energy required for a phase change and we frequently picture this as heating water on a stove or consumed by a freezer to make ice cubes. But, LHoV is not a fixed and constant value. Consider attempting to boil water at a high elevation or a liquid boiling in the vacuum of space. As ambient pressure changes, the partial pressure changes and the energy required to complete a phase change varies.
Our internal combustion engines are essentially air pumps with inclusion of a small quantity f fuel to achieve ignition. The fuel enters the carburetor with a given energy in its liquid phase. Inside the carburetor pressure drop is achieved through the venturi. At this reduced pressure the fuel has sufficient energy to achieve phase change and become a vapor. If we could achieve a sufficient drop in partial pressure then the fuel would draw heat from its environment but I believe this is not what we see.
Consider a CO2 fire extinguisher. The CO2 is stored as a gas under fairly high pressure. When we pull the trigger, the gas is released, pressure drops, and the nozzle will commonly frost. Since there is no phase change, LHoV isn't the driver. Of course there are many factors incuding ambient temperature and humidity levels.
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